If Christopher Walken were not such an experienced actor, he might well qualify as Rookie of the Year. Until he wont the New York Film Critics Awards last week for best supporting actor, he had been a fairly unfamiliar fact to
movie audiences, even though he had won numerous awards for his work on the stage.

But in ''The Deer Hunter,'' Mr. Walken does his first real movie star turn, bringing virtuoso technique and glamour to a character who is, in effect, the film's romantic hero. His performance is so utterly
persuasive that his own parents, after watching the film and its brutal finale, rushed off to a telephone to make sure their son was still in one piece.

Perched nervously in an armchair in his agent's apartment recently, Mr. Walken talked about ''The Deer Hunter'' with fondness and a touch of amazement. But he sounded surprisingly modest about his own accomplishments
in the film.

''On the stage I know what I'm doing,'' he said, ''but in films I have to depend on the kindness of strangers.'' Kindest of them all was Michael Cimino, the director, who astonished
the actor by casting him in the film's second leading role.

''I walked in and he asked me who I'd like to play,'' Mr. Walken recalled. ''I named about four of them, but I figured if I was lucky I might get to play Stan, the one that John Cazale played.
I never expected him to cast me as Nick. It was just too good a part.'' Mr. Cimino has since been even kinder, casting Mr. Walken with Kris Kristofferson in the director's next film, ''Johnson County
War.'' Ordinarily, Mr. Walken's manner combines an element of urbanity with unexpected street toughness. But this time he is going to ride a horse and shoot people.

However much physical exertion Mr. Cimino's next film demands of its actors, it is not likely to be as strenuous as his Vietnam war epic, ''The Deer Hunter.'' That five-month shooting schedule culminated
in a grueling stay in Thailand, where a terrifying torture sequence was staged. Mr. Walken and other actors, playing American prisoners of war, were confined in special cages that had been built on the River Kwai, not far from
the famous bridge. As Mr. Walken recalls: 'The circumstances were genuine: We were up to here in water, it was hot, and we'd been doing it a long time. We'd been tied up. There were mosquitoes. There were rats.''

Then the director and Robert DeNiro, who stars in the film, came up with the idea of having one of the Thai actors repeatedly slap Mr. Walken's and Mr. DeNiro's faces, trying to force them to play a round of Russian roulette.
Mr. Walken had not known exactly how he was going to summon up the necessary note of horror for the scene - ''that was one of many big moments I had in the film, where I walked onto the set thinking, 'I wish
I knew what I was doing,''' he recalled. ''But when somebody belts you 50 times, you don't have to fake a reaction. You don't have trouble shaking.''

The P.O.W. scene was only one of many in which Mr. Walken says Mr. DeNiro helped him a great deal. ''He's the most generous actor I've ever worked with,'' he explained. In the scene where I'm
in the gaming room for the first time, for instance, and I pick up the gun and hold it to my head, he showed me how to do that. It's a very good moment for me, and I have to admit it's stolen.''

But Mr. Walken put his respect for Mr. DeNiro to good use. ''My admiration for him is one of the things that shows in the film, but it had some bearing on the characters. They're supposed to have been friends for
20 years, there's a powerful feeling between them. I think my feelings about his work help create an impression of warmth, of friendship.'' Some viewers of ''The Deer Hunter'' maintain that
the friendship has homosexual overtones, but Mr. Walken says that as far as he knows neither he nor the other principals had any such thing in mind.

Mr. Walken's performance, at first genial and then terrifyingly blank, is very much at odds with the coolly impassive characters he has played in other films. In ''Annie Hall,'' he was memorably stony as
Annie's very strange brother, and in ''Next Stop, Greenwich Village'' his unresponsiveness to the other characters amounted to a kind of cruelty.

''Sometimes people will look at that and say, 'What an economical actor.' But if I was to be frank about it, it was probably because I was nervous about being in movies.''

He hopes that that will change, and that his composure on screen will equal his confidence on the stage. Mr. Walken grew up in Astoria, Queens, where his father was a baker; his mother encouraged him and his two brothers to act.

While his brothers concentrated on television work, he went into musicals, first appearing in ''Best Foot Forward'' with another talented neophyte, Liza Minnelli. He landed a lot of chorus work after that, but
it was not until he won a leading role in ''The Lion in Winter'' that he was offered serious roles. ''The Lion in Winter'' called for him to wear tights, and he wore them so effectively
that a chance to play Romeo came his way. ''I was terrible,'' he notes. All told, he has about 60 plays to his credit, most notably ''Sweet Bird of Youth'' opposite Irene Worth.

Mr. Walken, who appears to be in his late 20's, is 35 years old. ''People always tell me to say I'm younger - better to have 'em think you're 30, that you haven't been around so long, that you're
not so old and work out.'' In fact, he looks younger and healthier in ''The Deer Hunter'' than he has in his previous films, which also include ''The Anderson Tapes'' and ''Roseland.''
As he explains it: Since I did those films, my life has changed to some extent. I don't drink as much as I used to, I take better care of myself. I tried running for a while, too, but I couldn't take it.
I figured I'd wind up having a great heart and dying of boredom.''

Mr. Walken, who lives in Manhattan with his wife, does not maintain that stage acting is intrinsically more worthwhile than screen acting. He would like to continue to do both. ''I love to make movies - now that I've
started to make 'em, I want to make a lot. I like the whole atmosphere. And also, I admire some of those actors so much, I want to learn how to do it.'' Until now, movie roles have been hard for him to come by,
and even now he is asked to audition for film directors. In the theater, his work is sufficiently well known and admired to preclude that.

With filmmakers, though, ''what they mostly do is say: 'Come sit. Speak, so we can hear you speak. Move your face.' That's O.K. with me. I don't mind doing it. I still want to get jobs. But I'm
looking forward to the day when I don't have to pretend I'm enjoying myself.''

Captured by the enemy, Nick, played by Christopher Walken, thinks longingly of his four good buddies and his home town thousands of miles away in "The Deer Hunter." (Universal Studios)